1. Field of the Invention
The present invention relates generally to semiconductor fabrication, and more particularly, to apparatuses and methods for using a Faraday shield in direct exposure to a plasma within an inductively coupled plasma etching apparatus.
2. Description of the Related Art
In semiconductor manufacturing, etching processes are commonly and repeatedly carried out. As is well known to those skilled in the art, there are two types of etching processes: wet etching and dry etching. Dry etching is typically performed using an inductively coupled plasma etching apparatus.
FIG. 1A shows an inductively coupled plasma etching apparatus 100, in accordance with the prior art. The inductively coupled plasma etching apparatus 100 includes an etching chamber structurally defined by chamber walls 101 and a window 111. The chamber walls 101 are typically fabricated from stainless steel or aluminum. The window 111 is typically fabricated from quartz. The chamber walls 101 and the window 111 are configured to form a chamber internal cavity 102.
A chuck 117 is positioned within the chamber internal cavity 102 near the bottom inner surface of the etching chamber. The chuck 117 is configured to receive and hold a semiconductor wafer (i.e., “wafer”) 119 upon which the etching process is performed. The chuck 117 can be electrically charged using an RF power supply 123. The RF power supply 123 is connected to matching circuitry 121 through a connection 127. The matching circuitry 121 is connected to the chuck 117 through a connection 125. In this manner, the RF power supply 123 is connected to the chuck 117.
A coil 133 is positioned above the window 111. The coil 133 is fabricated from an electrically conductive material and includes at least one complete turn. The exemplary coil 133 shown in FIG. 1A includes three turns. The coil 133 symbols having an “X” indicate that the coil 133 extends rotationally into the page. Conversely, the coil 133 symbols having a “•” indicate that the coil 133 extends rotationally out of the page. An RF power supply 141 is configured to supply RF power to the coil 133. In general, the RF power supply 141 is connected to matching circuitry 139 through a connection 145. The matching circuitry 139 is connected to the coil 133 through a connection 143. In this manner, the RF power supply 141 is connected to the coil 133. A Faraday shield 149 is positioned between the coil 133 and the window 111. The Faraday shield 149 is maintained in a spaced apart relationship relative to the coil 133. The Faraday shield 149 is disposed immediately above the window 111. The coil 133, the Faraday shield 149, and the window 111 are each configured to be substantially parallel to one another.
FIG. 1B shows the basic operating principles of the inductively coupled plasma etching apparatus 100, in accordance with the prior art. During operation, a reactant gas flows through the chamber internal cavity 102 from a gas lead-in port (not shown) to a gas exhaust port (not shown). High frequency power is then applied from the RF power supply 141 to the coil 133 to cause an RF current to flow through the coil 133. The RF current flowing through the coil 133 generates an electromagnetic field 151 about the coil 133. The electromagnetic field 151 generates an inductive current 153 within the chamber internal cavity 102. The inductive current 153 acts on the reactant gas to generate a plasma 155. High frequency power is applied from the RF power supply 123 to the chuck 117 to provide directionality to the plasma 155 such that the plasma 155 is “pulled” down onto the wafer 119 surface to effect the etching process. An electrostatic field is also generated between the coil 133 and the plasma 155. This field is not necessarily uniform. High voltage gradients can drive the plasma 155 into the window 111 with sufficient energy to erode the window 111, and cause large temperature gradients within the window 111. The Faraday shield 149 ensures the electrostatic field is more uniformly distributed across the window 111, thus lessening the effects of temperature and erosion.
The plasma 155 contains various types of radicals in the form of positive and negative ions. The chemical reactions of the various types of positive and negative ions are used to etch the wafer 119. During the etching process, the coil 133 performs a function analogous to that of a primary coil in a transformer, while the plasma 155 performs a function analogous to that of a secondary coil in the transformer.
The reaction products generated by the etching process may be volatile or non-volatile. The volatile reaction products are discarded along with used reactant gas through the gas exhaust port. The non-volatile reaction products, however, typically remain in the etching chamber. The non-volatile reaction products may adhere to the chamber walls 101 and the window 111.
FIG. 1C shows an illustration of a deposition 157 of non-volatile reaction products on the window 111 in accordance with the prior art. Adherence of non-volatile reaction products to the window 111 may interfere with the etching process. Excessive deposition 157 may result in particles flaking off the window 111 onto the wafer 119, thus interfering with the etching process. Excessive deposition 157, therefore, requires more frequent cleaning of the chamber walls 101 and the window 111 which adversely affects wafer 119 throughput.
In contrast to the deposition 157 of non-volatile reaction products on the window 111, plasma 155 sputter can cause erosion of the window 111. FIG. 1D shows an illustration of window 111 erosion 159 in accordance with the prior art. Such erosion 159 not only shortens the useful lifetime of the window 111, but also generates particles which can contaminate the wafer 119 and introduce unwanted chemical species into the chamber internal cavity 102. The presence of unwanted species in the chamber internal cavity 102 is particularly undesirable because it leads to poor reproducibility of the etching process conditions and resulting wafer 119 characteristics.
In addition to the deposition 157 and erosion 159 problems associated with the window 111, selection of the window 111 material is limited by the thermal output of the etching process. During the etching process, the window 111 is exposed directly to the plasma 155. Therefore, the window 111 must absorb not only the heat generated by the bulk plasma 155 but also the heat transferred to the window 111 from sputtered plasma 155. The thermal properties of the window 111 must be sufficient to accommodate the thermal energy absorbed by the window 111 during the etching process. The thermal properties of the window 111 are primarily defined by the window 111 material.
Quartz is commonly used as a window 111 material in the inductively coupled plasma etching apparatus 100. The primary benefit associated with quartz is its low coefficient of thermal expansion. Thus, in the presence of a high temperature gradient from its center to its edge, the quartz window 111 will not experience differential thermal expansion leading to cracking and failure. Quartz, however, has a relatively low tensile strength. Thus, a large (e.g., ≧1.75 inch) quartz window 111 thickness is typically required to span the opening above the chamber internal cavity 102. The quartz window 111 is relatively expensive and costly to replace upon failure. Thus, it is desirable to have more flexibility in using window 111 materials other than quartz.
Ceramic has been used as an alternative to quartz for the window 111 material. Ceramic is more durable, stronger, and less expensive that quartz. However, ceramic materials have a higher coefficient of thermal expansion than quartz. Thus, when exposed to a high thermal output associated with certain etching processes, the ceramic window 111 is more susceptible to experiencing differential thermal expansion leading to cracking and failure. For ceramic window 111 materials to be used in higher thermal output etching processes, it is necessary to maintain a low temperature gradient across the ceramic window 111 to prevent cracking and failure.
In view of the foregoing, there is a need for an apparatus and a method to protect the window from deposition of non-volatile reaction products, erosion due to plasma sputter, and high temperatures resulting from the heat source associated with the etching process.